Turbine shafts are generally used in turbomachines. A steam turbine may be considered as an example of a turbomachine. To increase efficiency, steam turbines are designed as what are known as combined steam turbines. Steam turbines of this type have an inflow region and two or more flow regions designed with rotor blades and guide vanes. A flow medium flows via the inflow region to a first flow region and then to a further flow region. Steam may be considered as an example of a flow medium in this context.
By way of example, steam is passed into the inflow region at temperatures of over 400° C. and from there passes to the first flow region. In this case, various components, in particular the turbine shaft, are subject to thermal loads in the first flow region. Downstream of the first flow region, the steam flows to the second flow region. The steam is generally at lower temperatures and pressures in the second flow region. The turbine shaft should have properties of being tough at low temperatures in this region.
Various solutions have hitherto been disclosed for combining the two required properties of the turbine shaft with one another. One solution provides for the heat-resistant property and the property of being tough at low temperatures to be combined with one another in the turbine shaft. In this case, what is described as a monobloc shaft which combines the two required properties with certain restrictions is used. However, this involves compromises which can lead to restrictions on the design and operation of the steam turbine.
It is also known to weld turbine shafts. In the case of the materials which have been disclosed hitherto, with the associated demands imposed thereon, a buffer weld has to be applied to a material, which has to be annealed at a set temperature. After the annealing of the buffer weld on a first material, the two parts of the turbine shaft made from a first material and a second material are joined by a structural weld with a final tempering treatment at a temperature which is lower than the temperature used during the annealing of the buffer weld. Hitherto, 1% CrMoV has been used as material for the first region of the turbine shaft, which needs to have heat-resistant properties. Hitherto, 3.5% NiCrMoV has been used for the second region of the turbine shaft, which has to be tough at low temperatures.
The process for producing turbine shafts of this type is expensive and complicated.